RFID with multiple antennas

30 Oct 2012

I am prototyping an RFID system that would use multiple antennas and a single reader. The entire system would be controlled by mBed. The problem that i am facing is, how to mutiplex the various antennas with the single reader. The reader is using 125KHz and the antennas are simple coils.

Initially i tried a mutiplexer (https://www.sparkfun.com/products/299) but there seems to be very poor isolation. The RFID reader picks up antennas that are actually off. i.e the mux leaks signals into the channels are in OFF state. (i guess this is due to stray capacitances in the MUX Ic)

Doing a bit of googling yielded something called RF switches, which i guess are meant for antenna switching. They promise good isolation and cross-talk inhibition.

A few examples are: http://www.analog.com/en/rfif-components/rf-switches/adg901/products/product.html http://www.nxp.com/products/rf/switches/SA630D.html#overview

Now, can someone tell me if i am on the right track here? i.e using these RF switches to mutiplex several antennas to my single reader. I do not want to end up ordering a bunch of material that wont be useful.

Any help is greatly appreciated.

30 Oct 2012

At 125KHz, you want to look for analog or video/audio mux chips that have very high "off" state isolation. That done, the PC board layout and wiring are a real challenge at that frequency. The transmission line to the antenna is very tricky at that low frequency where the E field and H fields get confused.

some PC design and chip that gives you like 80dB of isolation might do. If you can figure out the transmission line issue.

Probably easier to use a reader per antenna.

02 Nov 2012

Thanks for your input steve. Yes, i am looking into some high-isolation RF switches. I managed to find one that is suitable for me (Analog device ADG751).

PCB design will be challenging too, i will use per reader antenna as a last resort.

05 Dec 2012

You can use a small relay. Also switch the unused antenna to ground with the contacts.

05 Jul 2016

Hi Kailash,

Did you ever get this working? I'm going to try using relays for multiple antennae at 134 kHz using a Raspberry pi.

Any info you can give much appreciated. Cheers Lawrence